Generally, X-ray tubes are used for medical use, dental use or the like as X-ray diagnosis. In this type of X-ray tube, a cathode and an anode made of copper are disposed in a vacuum envelope such that they face each other. An anode is provided with an anode rod, which extends out of the vacuum envelope. When a high voltage is applied between the cathode and the anode, the electron emission source of the cathode emits electrons. These electrons collide with the anode target, from which X-rays are radiated. Heat generated in the anode along with the X-ray radiation is released to the outside of the vacuum envelope via the anode rod.
The difference between the thermal expansion coefficient of copper forming the anode and the thermal expansion coefficient of the glass forming part of the vacuum envelope is large. For this reason, in a case where the vacuum envelope is formed, not only glass but also a metal such as Kovar (KOV) having a thermal expansion coefficient substantially equal to that of glass are used. In other word, the vacuum envelope includes a glass container and a metal member. The metal member is, for example, annular and is used as a joint between the glass container and the anode.
Heat transmitted through the anode rod is transferred to, for example, a molding material or the like covering surroundings of the X-ray tube. Further, for the purpose of effectively cooling the anode, it is also conceivable to attach a radiator having a diameter larger than that of the columnar anode rod.
For example, as the above-mentioned molding material, in order to improve the thermal conductivity, it is possible to use a material in which fine particles of inorganic substance are mixed in silicone. Further, it is also possible to use as a molding material a mixture of inorganic fine particles composed of substances such as lead, tungsten, tantalum, bismuth, barium and the like having a large number of molecules in silicone. In this case, the X-ray shielding function can be imparted to the molding material.
As described above, for the purpose of imparting the thermal conductivity and the X-ray shielding function to the molding material, in a case where fine particles of inorganic substance are mixed in a resin material such as silicone in order to improve thermal conductivity and X-ray shielding performance, the thermal expansion coefficient of the molding material is more or less smaller than the thermal expansion coefficient of the silicone alone. The thermal expansion coefficient of the silicone alone is 270×10^-6/K. In a case where fine particles are mixed, the thermal expansion coefficient is about 200 to 270×10^-6/K. On the other hand, the thermal expansion coefficient of copper constituting the anode (anode rod) is 16.5×10^-6/K.
For example, the anode rod is fixed to the housing, and the molding material is charged into the housing. In order to cure the molding material, it is necessary to heat the molding material charged into the housing and before curing to a high temperature. At that time, the anode rod and the molding material expand with their thermal expansion coefficients, and the molding material cures in the expanded state. As members of the X-ray tube shrinks at a different shrinkage percentage at the time of subsequent cooling, the molding material generates stress acting on the X-ray tube. This stress is alleviated as the temperature rises during use of the X-ray tube, but it occurs again when the X-ray tube is cooled. In addition, stress may also be generated by volume shrinkage accompanying curing of the molding material. This volume shrinkage increases as the amount of volatile diluent added to the resin of the molding material increases.
When this stress concentrates in the vicinity of the metal member connecting the X-ray tube and the glass container, breakage of the glass container, and separation between the metal member and the glass container occur, and the vacuum airtightness of the X-ray tube may not be maintained.